The use of vehicle headlights both by day and by night has two functions. Obviously, the lights assist the driver of the vehicle primarily when driving at night or in shaded locations such as bridges, tunnels and the like.
Another important aspect of the use of headlights is that they assist oncoming drivers and other road users such as pedestrians in locating an oncoming vehicle both at night and by day. The primary function of headlights is obviously to enable the driver to see where he is going at night. However, the headlights of a vehicle also perform other useful functions in that they assist simply as a warning that a vehicle is approaching by night or by day. Headlights also as a means of judging speed of the approaching vehicle, and in many cases, and its location. The spacing of the lights to some extent enable a person to judge the speed of an approaching vehicle.
Most vehicles have two headlights spaced apart on either side of the front of the vehicle. As in the case of any electrical device, headlights may fail. Bulbs may burn out. Wiring may break. A light may be damaged in a minor collision.
In any of these cases, the vehicle is then left with only one light, or in extreme cases, with none at all. If this happens, then where there is only one light which is lit, a pedestrian, or the driver of another oncoming vehicle may have no way of judging the width of the vehicle with the defective light. He may misjudge the location of the defective vehicle on the road and a collision results. Other mishaps may occur as will be apparent to the reader.
Where both headlights fail the situation is even more hazardous. Even if the street lights in the location of the vehicle are sufficient to enable the driver to drive slowly and cautiously to a place of safety, or where he can obtain assistance, the mere presence of an unlit moving vehicle will create great danger for other road users and pedestrians as well.
Often it is not possible for the driver of the defective vehicle to have the light repaired immediately. He may have to drive many miles, especially at night, before finding a service station where the problem can be repaired. The danger will exist as long as he drives the defective vehicle.
It is clearly desirable to have some form of temporary device which can help the driver travel at least some distance to reduce the danger of collision. Ideally, vehicles would be equipped with a portable light which could somehow be fastened in place of the defective light. However, so far such solutions have been impractical and, often, so expensive as to be prohibitive or far to bulky and delicate to remain unobtrusively in a vehicle glove or tool box until such time as they may be required.